1. Field of the Invention
The present invention relates to qualifying valid data read from a magnetic tape or disk system. More particularly, the present invention relates to qualifying valid data peaks occurring in a raw output signal of the type produced by a magnetic head used in a magnetic tape or disk drive storage device.
2. Description of the Prior Art
In a magnetic storage system such as a computer disk drive, digital information is magnetically stored at the surface of the disk. Digital information is represented by selectively polarizing consecutive areas across the surface of the rotating magnetic disk. When this information is read back from the storage disk, the magnetic polarization of the medium is sensed and converted into an electrical output signal: i.e. the raw data signal. The raw data signal is representative of the relative strength of the magnetic flux density present on the magnetic disk. This reading and writing operation is accomplished through a magnetic read/write head.
It is highly desirable to provide high levels of information storage densities on the magnetic disk.
One design criteria in a recording system is to provide an areal density as high as possible for the recording surface without compromising the performance of the system. Unfortunately, increased storage densities lead to significant noise levels in the raw data output signal. The source of this noise signal may include irregularities in the disk surface and the electromagnetic noise associated with proximate electrical devices. Furthermore, high density data coding schemes, such as the 1, 7 code, may further increase the susceptibility of the raw data output signal to unwanted noise. The 1, 7 code employs a single zero magnetic flux cell between any two adjacent flux cells and no more than seven adjacent flux cells. Codes that employ wide "windows" in order to increase disk capacity have larger band widths and longer baselines leading to noise and crosstalk susceptibility. This noise problem may be further exacerbated if the magnetic read head drifts slightly off track.
Data coding schemes which are intended to increase disk capacity and which have a minimum pulse separation of one provide reduced amplitude levels in closely spaced pulses due to inter-symbol interference. This decreases the signal-to-noise ratio, which forces the pulse qualification level to be lowered, and further adds to the off track noise problem.
Simple pulse qualification, in which the first peak over a threshold is taken to be the location of the data "pulse", is no longer adequate. The qualification level typically has been set so that there are an equal number of "dropouts" and an equal number of "extra" pulses due to the noise present in the raw data output signal. Coding schemes directed toward increased data densities, however, can be expected to have significantly more occasions where there are multiple peaks occurring above the lowered qualification threshold and/or peaks located in the long baseline between widely separated pulses. Threshold detectors used to disqualify data peaks whose amplitude does not exceed a prescribed level are not adequate to disqualify false data peaks when the ratio between the signal level and the noise level is small. A "first peak" qualification can be expected to place many pulse locations outside of the proper window. Not only does this cause data decoding problems, it also causes the phase lock loop used in the read back circuitry to "jitter."